Autonomic Neurons

As discussed in chapter 7, neurons of the peripheral nervous system (PNS) that conduct impulses away from the central nervous system (CNS) are known as motor, or efferent, neurons. There are two major categories of motor neurons: somatic and autonomic. Somatic motor neurons have their cell bodies within the CNS and send axons to skeletal muscles, which are usually under voluntary control. This was briefly described in chapter 8 (see fig. 8.23), in the section on the reflex arc, and is reviewed in figure 9.1a. The control of skeletal muscles by somatic motor neurons is discussed in depth in chapter 12.

Unlike somatic motor neurons, which conduct impulses along a single axon from the spinal cord to the neuromuscular junction, autonomic motor control involves two neurons in the efferent pathway (table 9.1). The first of these neurons has its cell body in the gray matter of the brain or spinal cord. The

Somatic motor reflex

Interneuron^ Dorsal root ganglion

Interneuron^ Dorsal root ganglion

Autonomic motor reflex

Interneuron Dorsal root ganglion

Autonomic motor reflex

Interneuron Dorsal root ganglion

■ Figure 9.1 Comparison of a somatic motor reflex and an autonomic motor reflex. In a skeletal muscle reflex, a single somatic motor neuron passes from the CNS to the skeletal muscle. In an autonomic reflex, a preganglionic neuron passes from the CNS to an autonomic ganglion, where it synapses with a second autonomic neuron. It is that second, or postganglionic, neuron that innervates the smooth muscle, cardiac muscle, or gland.

The Autonomic Nervous System axon of this neuron does not directly innervate the effector organ but instead synapses with a second neuron within an autonomic ganglion (a ganglion is a collection of cell bodies outside the CNS). The first neuron is thus called a preganglionic neuron. The second neuron in this pathway, called a postganglionic neuron, has an axon that extends from the autonomic ganglion to an effector organ, where it synapses with its target tissue (fig. 9.1ft).

Preganglionic autonomic fibers originate in the midbrain and hindbrain and in the upper thoracic to the fourth sacral levels of the spinal cord. Autonomic ganglia are located in the head, neck, and abdomen; chains of autonomic ganglia also parallel the right and left sides of the spinal cord. The origin of the preganglionic fibers and the location of the autonomic ganglia help to distinguish the sympathetic and parasympathetic divisions of the autonomic system, discussed in later sections of this chapter.

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